Regarding the February cover article on the quest for a laboratory geodynamo (Physics Today, February 2006, page 13), it is important, as Isaac Newton said, to “stand on the shoulders of giants” as we advance our understanding. Unfortunately, the present generation often fails to do so. Consider, for example, the Bullard–Rikitake dynamo theory, which explains not only the axial field but also its periodic spontaneous reversal, as observed in ocean-bottom cores. Here is the background:
In the early 1950s, Edward Bullard and a student of his named Rikitake built a geodynamo at the University of Newcastle upon Tyne in the UK. It consisted of two counter-rotating iron cylinders about two meters in diameter, connected electrically by an equatorial layer of mercury. It generated an axial magnetic field that spontaneously reversed its direction every 20 minutes, as Earth’s field is known to do every 10 000 years or so.
The actual geodynamo has yet another peculiar and unexplained property: It is substantially off-center by about 10% of Earth’s diameter. Earth’s field is about 0.6 gauss in Siberia, and about 0.1 gauss in the diametrically opposite region in the southern Atlantic Ocean, as I pointed out in a paper presented in the 1950s at a symposium at Newcastle. This asymmetry is considerably harder to explain than the field generation itself or its periodic reversals.
I mention this for readers who may be interested in joining this fascinating field of experimental geophysics.
